This invention relates to a new manufacturing method of printed circuit boards (PCB's) in general and of PCB's with plated-through holes in particular. It results in improved moisture resistance of PCB's.
In circuit boards where high accuracies and high impedances are required, for example, impedances greater than one (1) megohms as in digital multimeter PCB's, special precautions have to be taken to minimize current leakage paths in the PCB's by maintaining high impedances of the board. Typical special precautions taken are conformal coating, that is coating all components on the board with some insulating compound, and elevating from the board all the critical points in the circuit by means of insulator stand-offs.
In PCB's made of glass-epoxy phenolic or paper laminate, for instance, a major source of current leakage paths is moisture wicking in through the periphery of the board where capillary action through the exposed PCB edge causes the moisture wicking. This moisture wicking, in turn, causes current leakage paths within the PCB.
In the typical process of sizing the board, for example, the routing operation exposes raw fibers at the edges of a glass-epoxy board to the ambient environment. Thus, in a high moisture environment, the moisture wicking problem earlier described is especially severe, and the problem of current leakage is especially pronounced. To compound the problem, the usual special precautions of conformal coating and insulator stand-offs do not effectively address this problem of leakage paths caused by moisture wicking of the PCB.